To obtain acceptable ellipsometer and/or polarimeter performance over a wide spectral range, compensator-based ellipsometer and/or polarimeter designs require a compensator element that provides retardance within a certain acceptable range over the entire spectral range. Traditionally, birefringent waveplates of quartz or MgF2 have been used as compensator elements in rotating element designs. A single waveplate exhibits a (1/wavelength) dependence in retardance vs. wavelength, while a dual/multiple waveplate design, (as disclosed in U.S. Pat. No. 6,353,477), can minimize the effect of the (1/wavelength) dependence.
With the present invention in mind, known relevant patents are:                U.S. Pat. No. 5,706,212 to Thompson et al.;        U.S. Pat. No. 6,353,477 to Johs et al.;        U.S. Pat. No. 5,963,325 to Johs et al.;        U.S. Pat. No. 6,141,102 to Johs et al.;        U.S. Pat. No. 6,084,675 to Herzinger et al.;        U.S. Pat. No. 6,118,537 to Johs et al.;        U.S. Pat. No. 6,100,981 to Johs et al.;        U.S. Pat. No. 6,084,674 to Johs et al.        
An Application which published Jun. 28, 2007 was recently identified and is US 2007/0146706 A1.
Further, references identified by the EPO in examination of an Application corresponding to the U.S. application Ser. No. 12/290,787 are:                “Total Internal Reflection Phase Retarders Constructed from Prisms”, Nagib, J. Opt. A: Pure Appl., Opt 6 (2004), pp. 425-428;        “Phase Retarders Highly Insensitive to the Input Angle”, Nagib, Applied Optics, Vol. 37, No. 7, March 1998;        U.S. Pat. No. 4,917,461, to Goldstein;        U.S. Pat. No. 6,456,376, to Liphardt et al.        
While interesting, it is noted that the EPO has proceeded to allow Claims to prisms somewhat like disclosed in the Nagib articles, as application of said Nagib prisms failed to report any improvement in beam locus sensitivity to changes in prism translation. This is believed to be the result of the Nagib prisms having a length to width aspect ratio of over about 7.0. As disclosed in this Application, the presently Claimed Retarders have an aspect ratio of about 1.0. It is believed that the length of the prisms reported in the Nagib articles masked the beneficial effects achieved with present invention prisms.
Need remains for additional retarder systems which provide improved characteristics, and containment systems for securing them in intended positions, as well as protecting them from contamination for the environment.